Just flag a different post, choose Other and clearly explain the situation.
I have taken this approach before, and it was well-received: The flag was marked helpful, and the spamming user was indeed deleted:

Just flagged it this way. Let's see how things go.
–
AntonyApr 28 '13 at 10:54

2

I think they try to avoid nuking cross site spammers right away to try and catch more spammers from same IP address, then they cast IP ban. Single site spam is more simple case and lead to account deletion within few hours from what I've seen so far.
–
Shadow WizardApr 28 '13 at 11:04

The user is removed but the flag is still waiting for review. Yannis is right, I just did.
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AntonyApr 29 '13 at 11:35